June 20, 2014

Stream Of Gas Observed Escaping Grasp Of Supermassive Black Hole

The classic image of black hole systems is the in-flowing gas being consumed by the singularity. However, black holes are far from simply being cosmic vacuum cleaners, and occasionally even exhibit some strange behavior.

Recently, a team of astronomers found a clumpy stream of gas quickly flowing outward from the supermassive black hole at the core of the galaxy NGC 5548. This unusual phenomenon was discovered when a coordinated observing campaign found that up to 90 percent of the X-rays normally emitted by the black hole were blocked.

“This is a milestone in understanding how supermassive black holes interact with their host galaxies,” says Jelle Kaastra of the SRON Netherlands Institute for Space Research, who led the research team.

Using various observatories such as XMM-Newton, the Hubble Space Telescope, Swift, NuSTAR, Chandra, and INTEGRAL, the team was able to monitor the system in 2013 and 2014, the most extensive monitoring campaign ever undertaken.

Results from this work provide the first direct evidence for an effect that had long been predicted in astronomy.

"There are other galaxies that show gas streams near a black hole, but they haven't changed as dramatically. This is the first time we've seen a stream like this move into the line of sight," said Gerard Kriss of the Space Telescope Science Institute in Baltimore, Md. "We just happened to get lucky. With most objects like this, you don't normally see this kind of event."

Theoretical models indicated that a shielding process is necessary to accelerate powerful gas streams, commonly known as winds, to high speeds. However, we simply hadn’t been lucky enough to catch such interactions from the right angle.

The idea is that X-rays and ultraviolet light are created when matter, falling into a black hole, becomes heated by friction. The ultraviolet component of the light can create an outward wind from the singularity, accelerating matter outward that would have otherwise been consumed by the black hole. However, the winds only emerge if the point of origin is shielded from the X-ray component of the radiation.

The Seyfert Galaxy in question has been studied in detail for more than three years, but the signature shielding effect wasn’t observed until June of 2013.

"There were dramatic changes since the last observation with Hubble in 2011. I saw signatures of much colder gas than was present before, indicating that the wind had cooled down, due to a strong decrease of ionizing X-ray radiation from the nucleus," noted Kriss.

Once data from all six of the contributing observatories was combined and analyzed, the team was able to resolve the mechanism responsible for the evolving X-ray spectrum. The root cause was found to be an accelerating wind that had been observed previously to reach about 1,000 kilometers per second.

"The new wind reaches speeds of up to 5,000 kilometers per second but is much closer to the nucleus than the persistent wind," adds Kaastra. "The new gas outflow blocks 90 percent of the low-energy X-rays that come from very close to the black hole, and it obscures up to a third of the region that emits the ultraviolet radiation at a few light-days distance from the black hole."

This is confirmed by looking at optical data resolving regions farther from the nucleus. Since this area receives less radiation, cooling would be expected. According to co-author Nahum Arav, "Because of this cooling down, new features arise in the Hubble spectrum of the wind. These features allow us to pinpoint the location of the strongest persistent wind component."

While X-ray absorption has previously been observed in other systems, it hasn’t displayed the broadband characteristics seen in NGC 5548, and has therefore been attributed to other effects.

"However, in our case, thanks to the combined XMM-Newton and Hubble data, we know this is a fast stream of outflowing gas very close to the nucleus," added team member Massimo Cappi. Therefore, this study is viewed as the first of its kind.

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Image 2 (below): This artist’s impression illustrates the finding, by an international team of astronomers, of a clumpy gas stream flowing quickly outward from the supermassive black hole at the centre of active galaxy NGC 5548. Credit: NASA, ESA, and A. Feild (STScI)